Effect of Stocking Density and Dietary Protein Level in Biofloc System on the Growth, Digestive and Antioxidant Enzyme Activities, Health, and Resistance to Acute Crowding Stress in Juvenile Common Carp (Cyprinus carpio)

This study is aimed at evaluating the effects of stocking densities and dietary protein levels in the biofloc system on the performance of common carp. Fish ( 12.09 ± 0.99  g) were transferred to 15 tanks: fish reared at 10 kg/m3 as medium density and fed 35% (MD35) or 25% (MD25) protein and fish reared at 20 kg/m3 as high density and fed 35% (HD35) or 25% (HD25) protein in the biofloc system and control fish reared at MD and fed 35% protein in clear water. After 60 days, fish were subjected to crowding stress (80 kg/m3) for 24 h. The growth of fish was highest in MD35. The feed conversion ratio was lower in MD35 compared to the control and HD groups. The amylase, lipase, protease, and superoxide dismutase and glutathione peroxidase activities in the biofloc groups were significantly higher than in the control. After crowding stress, cortisol and glucose levels in biofloc treatments were significantly decreased compared to the control. After 12 and 24 h stress, lysozyme activity in MD35 was significantly lower than in the HD treatments. Overall, the biofloc system with MD could improve growth and robustness against acute stress in fish. Also, biofloc could compensate 10% reduction of protein in common carp juvenile diet when reared in MD.